.NS?- 108 18CHAPTER

STRATOSPHERICCHEMISTRY

0(10)

L .-_

N02

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Panel Members

R.A. Cox,

W.B. DeMore

E.E. FergusonR. Lesclaux

A.R. Ravishankara

Chairman

S.P. SanderN.D. Sze

R.Zellner

CHAPTER 2

STRATOSPHERIC CHEMISTRY

TABLE OF CONTENTS

2.0 INTRODUCTION ............................................................. 27

2.1 CURRENT STATUS OF KINETICS AND PHOTOCHEMICAL DATA BASE

FOR TRACE GAS FAMILIES INVOLVED IN OZONE CHEMISTRY ................. 29

2.1.10 x Chemistry ............................................................ 29

2.1.2 HO x Chemistry .......................................................... 30

2.1.3 NOx Chemistry .......................................................... 32

2.1.4 C10 x Chemistry .......................................................... 35

2.1.5 BrO x Chemistry .......................................................... 38

2.1.6 Sulfur Chemistry ......................................................... 40

2.1.7 Hydrocarbon Oxidation Chemistry .......................................... 40

2.1.8 Halocarbon Oxidation Chemistry ............................................ 42

2.2 SPECIAL ISSUES IN STRATOSPHERIC CHEMISTRY ............................ 43

2.2.1

2.2.2

2.2.3

2.2.4

2.2.5

2.2.6

2.2.7

2.2.8

Role of Reactions Involving Sodium Species .................................. 43

Ion Chemistry ........................................................... 44

Homogeneous Reactions Between Temporary Reservoir Species .................. 45

Heterogeneous Reactions .................................................. 46

Reactions With Complex Temperature and Pressure Functions ................... 48General Comments on Photodissociation Processes ............................. 50

Errors and Uncertainties in Kinetic and Photochemical Data ..................... 51

Identification of Gaps in the Chemical Description of the Atmosphere ............. 53

2.3 SUMMARY AND CONCLUSIONS .............................................. 54

STRATOSPHERIC CHEMISTRY

2.0 INTRODUCTION

Ozone is present in the earth's atmosphere at all altitudes from the surface up to at least 100 km.

The bulk of the ozone resides in the stratosphere with a maximum ozone concentration of 5 x 1012 molecule

cm -3 at about 25 km. In the mesosphere (> 60 km) Os densities are quite low and are not discussed in

the present report. Although O3 concentrations in the troposphere are also less than in the stratosphere,

ozone plays a vital role in the atmospheric chemistry in this region and also affects the thermal radiationbalance in the lower atmosphere.

Atmospheric ozone is formed by combination of atomic and molecular oxygen.

0 + 02 + M -- Os + M (1)

where M is a third body required to carry away the energy released in the combination reaction. At altitudes

above approximately 20 km production of O atoms results almost exclusively from photodissociation ofmolecular O2 by short wavelength ultraviolet radiation (X < 243 nm):

02 + hv _ O + O (2)

At lower altitudes and particularly in the troposphere, O atom formation from the photodissociation ofnitrogen dioxide by long wavelength ultraviolet radiation is more important:

NO2 + hv _ NO + O (3)

Ozone itself is photodissociated by both UV and visible light:

03 + hv _ 02 + O (4)

but this reaction together with the combination reaction (1) only serves to partition the 'odd oxygen' species

between O and O3. The production processes (2) and (3) are balanced by chemical and physical loss pro-

cesses. Until the 1950s, chemical loss of odd oxygen was attributed only to the reaction:

0 + 0 3 -- 0 2 + 0 2 (5)

originally proposed by S. Chapman (1930). It is now known that ozone in the stratosphere is removed

predominantly by catalytic cycles involving homogeneous gas phase reactions of active free radical species

in the HOx, NOx, C10 x and BrO x families:

X "{'- 0 3 _ XO + 0 2 (6)

XO + O -- X + Oz

net: O + 0 3 _ 202

(7)

where the catalyst X = H, OH, NO, C1 and Br. Thus these species can, with varying degrees of efficiency,

control the abundance and distribution of ozone in the stratosphere. Assignment of the relative importance

and the prediction of the future impact of these catalytic species is dependent on a detailed understanding

of the chemical reactions which form, remove and interconvert the active components of each family.

27

STRATOSPHERIC CHEMISTRY

This in turn requires knowledge of the atmospheric life cycles of the hydrogen, nitrogen and halogen-

containing precursor and sink molecules, which control the overall abundance of HO x, NO x and C10 x

species.

Physical loss of ozone from the stratosphere is mainly by dynamical transport to the troposphere where

further photochemically driven sources and sinks modify the ozone concentration field. Ozone is destroyedat the surface of the earth and so there is an overall downward flux in the lower part of the atmosphere.

Physical removal of ozone and other trace gaseous components can also occur in the precipitation elementsand on the surface of atmospheric aerosols. Since most of the precursor and sink molecules for the species

catalytically active in ozone removal in the stratosphere are derived from or removed in the troposphere,

global tropospheric chemistry is a significant feature of overall atmospheric ozone behavior.

Numerical simulation techniques are used to describe and investigate the behavior of the complex

chemical system controlling atmospheric composition, the models having elements of chemistry, radia-

tion and transport. The chemistry in such models may include some 150 elementary chemical reactions

and photochemical processes involving some 50 different species. Laboratory measurements of the ratesof these reactions have progressed rapidly over the past decade and have given us a basic understanding

of the kinetics of these elementary processes and the way they act in controlling ozone. This applies par-

ticularly in the upper stratosphere where local chemical composition is predominantly photochemicallycontrolled.

It has proved more difficult to describe adequately both the chemistry and the dynamics in the lower

stratosphere. Here the chemistry is complicated by the involvement of temporary reservoir species suchas HOC1, H202, HNO3, HCI, HNO4, N205 and CIONO2 which 'store' active radicals and which strongly

couple the HO x, NOx and C10 x families. The long photochemical and thermal lifetimes of ozone and the

reservoir species in this region give rise to strong interaction between chemistry and dynamics (transport)in the control of the distribution of ozone and other trace gases. Moreover, seasonal variability and natural

perturbations due to volcanic injections of gases and aerosol particles add further to complicate the descriptionand interpretation of atmospheric behavior in this region. Most of the changes in the predicted effects

of chlorofluoromethanes and other pollutants on ozone column density have resulted from changes in our

view of the chemistry in the lower stratosphere. A great deal of importance must therefore be attached

to achieving an understanding of the key factors in ozone chemistry in this region of the atmosphere.

Description of atmospheric chemistry, in the troposphere is similarly complicated by dynamical influence

and additionally by involvement of the precipitation elements (i.e. cloud, rain and snow) in the chemical

pathways. The homogeneous chemistry of the troposphere is centered round the role of the hydroxyl radical

in promoting oxidation and scavenging of trace gases released from surface terrestrial sources. Tropospheric

OH is an important issue for stratospheric ozone since it controls the flux of source gases such as CH 4,

halogenated hydrocarbons, and sulfur compounds to the stratosphere. Although the mechanisms are more

complex due to the involvement of larger and more varied entities, the overall pattern of relatively rapid

photochemical cycles involving a coupled carbon/hydrogen/nitrogen and oxygen chemistry is similar to

that in the stratosphere. The photochemical cycles influence both the odd hydrogen budget and also, throughcoupling of the hydrocarbon oxidation with NO2 photochemistry, the in situ production and removal of

tropospheric ozone. The concentration and distribution of tropospheric ozone is important in respect of

its significant contribution to the total ozone column, and its radiative properties in the atmospheric heat

balance. A detailed description of tropospheric chemistry is given in Chapter 4.

The numerical models employed to investigate atmospheric behavior require the best available input

data. Provision of an evaluated photochemical and kinetics data base for modelling atmospheric chemistry

28

STRATOSPHERIC CHEMISTRY

and ozone perturbations, has been recognized as an important feature of atmospheric programmes for some

years now. With the rapid growth in the amount of information and expertise available in recent years

this has become even more important. The evaluated data base produced by the NASA panel for Data

Evaluation, updated in February 1985 (NASA, 1985) is provided in Appendix A of this assessment. An

updated evaluation, containing more detailed presentation of the available data, has been published by